This invention relates to an infrared absorption gas detector in which the amount of infrared radiation absorbed by a gas mixture gives an indication of the presence of particular components within that gas.
The present invention is particularly concerned with infrared absorption gas detectors which are used to detect the presence of a particular gas, and, for example, trigger an alarm when the concentration of that gas exceeds a predetermined value. Such gas detectors can be used to detect a wide variety of gases the presence of which represents a hazard or danger. Such gases include common non-life supporting gases like carbon dioxide, toxic gases such as carbon monoxide or hydrogen sulphide, and flammable gases such as those in the methane series, methane, ethane, propane, butane, and vapours of flammable liquids.
At present, infrared absorbing gas detectors have monitoring heads which fall into two different and separate categories. Firstly, there are infrared absorption gas detectors which have a monitoring head using a double beam in space system in which two beams of infrared radiation traverse two different and separate paths in space through an atmosphere to be monitored and these two beams are filtered along their path so that they have different wavelengths. Both beams are usually derived from the same source but then follow different paths through the atmosphere to be monitored, and the resulting intensities of the two beams are detected by two independent radiation detectors. One of the beams serves as a reference beam by which allowance can be made for variations in the radiation output of the source and for changes in the sensitivity of the radiation detectors as a result of, for example, changes in ambient temperature while the other beam is the one which is absorbed by the gas to be detected. However, since two separate radiation detectors are used, difficulties are caused when the two detectors are not perfectly matched. Even if the detectors are matched reasonably well, differences still occur as a result of the ageing properties of the two radiation detectors and hence their operating conditions may also be different, for example, their temperatures may be different. Also no allowance can be made for any change in the transmission of the optical paths extending between the source and the detectors since the two beams travel over essentially different optical paths. The transmission of the two optical paths changes in use due the accretion of dust or debris on any surface in the optical path, and since the two optical paths are substantially independent through the atmosphere to be monitored any non-uniform accretion of dust or debris causes an imbalance between the two paths.
Often such double beam in space systems also include a chopper wheel which interrupts the output of the source to provide a pulsed beam of radiation. The chopper wheel may also be used to control the radiation emitted by the source and direct it along one path or the other. This is a further disadvantage since moving parts tend to wear, leading to unreliability. They also lead to the gas detector not being as robust as possible.
An example of this type of gas detector is described in U.S. Pat. No. 4,320,297, wherein the two beams follow two separate paths through the same sample gas cell and a chopper wheel is also included in this example of detector to interrupt the beam.
Secondly, the other conventional type of infrared gas absorption detector has a monitoring head which uses a double beam in time system whereby two beams of different wavelength alternately travel over the same optical path and are detected using the same radiation detector. This type of gas detector must include some means to change the wavelength of the beam and, normally, this has the form of a filter wheel which interposes spectral filters having different pass bands into the optical path between the source and the radiation detector. The filter wheel also acts as a chopper wheel to provide a pulsed source of radiation. Naturally, such an infrared absorption gas detector does permit some allowance to be made for changes in the transmission of all of the common parts of the optical path i.e. changes in transmission of all parts except the spectral filters, and thus allowance can be made for the non-uniform accretion of dust on surfaces of the optical path. The main disadvantage with this type of gas detector is the need for the means to introduce different spectral filters in the optical path which requires the gas detector to include moving parts and this leads to wear and unreliability, and thus to the gas detector not being as robust as possible. One example of a detector of this type is described in U.S. Pat. No. 4,358,679.
U.S. Pat. No. 3,539,804 discloses a small and compact gas detector for detecting carbon dioxide. This gas detector is essentially a double beam in space system. The monitoring head of the gas detector includes a single source of radiation, two detectors for detecting infrared radiation, an optical path extending between the source and the detectors, which includes a gas sample volume, and a selectively transmitting spectral filter located in the optical path leading to at least one of the detectors. In this example the source of infrared radiation is a conventional pre-focus bulb with a tungsten filament which provides a non-pulsed, concentrated but diverging beam of infrared radiation through the gas sample volume. The two detectors receive radiation that has passed through two different, but closely spaced, regions of the gas sample volume since the output beam of the source of infrared radiation diverges slightly. The radiation which impinges on one of the detectors thus forms a reference beam and that impinging on the other the analytical beam, and the filter located in front of at least one of them modifies the wavelength of that part of the radiation to provide the difference in wavelength between the beams.
Although in this example the two detectors receive radiation from adjacent portions of the gas sample volume, there are essentially still two separate and independent beams and thus this example has all the limitations of the conventional double beam in space system and any uneven contamination of the bulb and pre-focus lens or any contamination of a downstream window of the gas detector leading to the infrared radiation detectors and filters leads to an imbalance in the system. Indeed the disclosure in U.S. Pat. No. 3,539,804 does discuss the inclusion of a neutral density filter covering half the entry window to the infrared radiation detector assembly and how, by rotating this, it is possible to cause an imbalance between the two beams to try to match the response of the two radiation detectors. However, such a method can only be used during the initial set up of the instrument and does not take any account of the changes in the contamination of the monitoring head during use. This patent also discloses that some of these problems in the system can be overcome by moving the selectively transmitting spectral filter into and out of the optical path between the source and one of the radiation detectors, and so convert this apparatus into one operating as a double beam in time system.